The present invention relates generally to energy conversion, and particularly to a process and apparatus for production of purified hydrogen by steam reforming.
Purified hydrogen is an important fuel source for many energy conversion devices. For example, fuel cells use purified hydrogen and an oxidant to produce an electrical potential. A process known as steam reforming produces by chemical reaction hydrogen and certain byproducts or impurities. A subsequent purification process removes the undesirable impurities to provide hydrogen sufficiently purified for application to a fuel cell.
Under steam reforming, one reacts steam and alcohol, (methanol or ethanol) or a hydrocarbon (such as methane or gasoline or propane), over a catalyst. Steam reforming requires elevated temperature, e.g., between 250 degrees centigrade and 800 degrees centigrade, and produces primarily hydrogen and carbon dioxide. Some trace quantities of unreacted reactants and trace quantities of byproducts such as carbon monoxide also result from steam reforming.
Trace quantities of carbon monoxide, certain concentrations of carbon dioxide, and in some cases unsaturated hydrocarbons and alcohols will poison a fuel cell. Carbon monoxide adsorbs onto the platinum catalyst of the fuel cell and inhibits operation of the fuel cell, i.e., reduces the power output of the fuel cell. To a lesser degree, carbon dioxide and other unsaturated hydrocarbons and alcohols have the same result. All impurities to some extent reduce by dilution the partial pressure of hydrogen in the fuel cell and increase the mass transfer resistance for hydrogen to diffuse to the platinum catalyst, and thereby reduce power output of the fuel cell. Thus, fuel cells require an appropriate fuel input, i.e., purified hydrogen with no additional elements contributing to a loss in fuel cell efficiency.
Traditionally, hydrogen purification attempts to always maximize harvest of hydrogen from the reforming process. To maximize the amount of hydrogen obtained, a relatively expensive device, e.g., a thick and high quality palladium membrane, serves as a hydrogen-permeable and hydrogen-selective membrane [Ledjeff-Hey, K., V. Formanski, Th. Kalk, and J. Roes, xe2x80x9cCompact Hydrogen Production Systems for Solid Polymer Fuel Cellsxe2x80x9d presented at the Fifth Grove Fuel Cell Symposium, Sep. 22-25, 1997]. Such thick, high quality palladium alloy membranes support maximum harvest of hydrogen with minimal, i.e., acceptable, impurities for use in a fuel cell. Such high level of purification, however, requires significant investment in the thick, high quality palladium membrane.
Traditionally, the process of steam reforming and the subsequent process of hydrogen purification occur in separate apparatus. The advantages of combining steam reforming and hydrogen purification in a single device are known [Oertel, M., et al, xe2x80x9cSteam Reforming of Natural Gas with Integrated Hydrogen Separation for Hydrogen Productionxe2x80x9d, Chem. Eng. Technol 10 (1987) 248-255; Marianowski, L. G., and D. K. Fleming, xe2x80x9cHydrogen Forming Reaction Processxe2x80x9d U.S. Pat. No. 4,810,485, Mar. 7, 1989]. An integrated steam reforming and hydrogen purification device should provide a more compact device operating at lower temperatures not limited by the normal equilibrium limitations. Unfortunately, such a device has yet to be reduced to practical design. Where theory in this art recognizes the advantage of combining steam reformation and hydrogen purification in a single device, the art has yet to present a practical, i.e., economical, design.
Thus, a practical integrated steam reforming and hydrogen purification device has not yet become available. The subject matter of the present invention provides a practical combined steam reforming and hydrogen purification device.
A process for producing hydrogen containing concentrations of carbon monoxide and carbon dioxide below a given level begins by reacting an alcohol vapor (such as methanol) or a hydrocarbon vapor (such as propane) and steam to produce product hydrogen, carbon monoxide, and carbon dioxide. The reacting step occurs in the vicinity of, or immediately preceding, a hydrogen-permeable and hydrogen-selective membrane and the product hydrogen permeates the membrane. A methanation catalyst bed lies at the permeate side of the membrane and converts any carbon monoxide and carbon dioxide which passes through the membrane to methane, thereby yielding a product hydrogen stream with concentrations of carbon monoxide and carbon dioxide that are below acceptable thresholds. Optionally, reforming catalyst may also lie at the permeate side of the membrane along with the methanation catalyst to convert to product hydrogen any unreacted alcohol or hydrocarbon feed that passes through the membrane. Product hydrogen is then withdrawn from the methanation catalyst bed.
A steam reformer, also referred to as a fuel processor, according to the present invention includes a reforming bed that receives and reacts a mixture of alcohol or hydrocarbon vapor and steam to produce hydrogen and by product gases. The gases are then passed through a hydrogen-permeable and hydrogen selective membrane. On the permeate side of the membrane, a methanation catalyst converts carbon monoxide and carbon dioxide to methane.
Many other features of the present invention will become manifest to those versed in the art upon making reference to the detailed description which follows and the accompanying drawings in which preferred embodiments incorporating the principles of this invention are disclosed as illustrative examples only.